The article by Heppenheimer (Jan. 29), "It Is Time to Call the Shuttle Obsolete," is misleading. In his analogy of the explosion of the Challenger and that of the dirigible Hindenburg, the author states that the Hindenburg disaster "exposed the flaws in the dirigible as a passenger carrier."
This is not entirely true. The Hindenburg explosion was caused by the fact that the dirigible was filled with highly explosive hydrogen rather than inert helium. This unfortunate situation was because the only source of helium in 1937 was the United States and, for various reasons, we refused to sell it to the Germans.
JOHN H. MARTIN